Subclass of ModelMap that implements the Model interface.
Java 5 specific like the Model interface itself.

This is an implementation class exposed to handler methods by Spring MVC, typically via
a declaration of the Model interface. There is no need to
build it within user code; a plain ModelMap or even a just
a regular Map with String keys will be good enough to return a user model.

addAttribute

Note: Empty Collections are not added to
the model when using this method because we cannot correctly determine
the true convention name. View code should check for null rather
than for empty collections as is already done by JSTL tags.